Answer 3.3

In 1961, the unified atomic mass [u] has been defined as 1/12 of the mass of one atom of nuclide 12C which has been assigned to 12 u exactly; 1 u = 1.66055 * 10–27 kg.

b) What is the difference between nominal mass, isotopic mass, and relative atomic mass?

The nominal mass is defined as the integer mass of the most abundant naturally occurring stable isotope of an element. The nominal mass of an element is often equal to the integer mass of the lowest mass isotope of that element, e.g., for H, C, N, O, S, Si, P, F, Cl, Br, I. The nominal mass of an ion is the sum of the nominal masses of the elements in its empirical formula.

The isotopic mass is the exact mass of an isotope. It is very close to but not equal to the nominal mass of the isotope.

The relative atomic mass or the atomic weight as it is also often imprecisely termed is calculated as the weighted average of the naturally occurring isotopes of an element.

c) How do you calculate the relative atomic mass (atomic weight) from isotopic masses and abundance values?

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